[17.0] auto_backup - fail or disk space alert

[rpinset]

Is your feature request related to an issue?
No

Describe the solution you want
notify subscribers in the event of failure and/or missing disk space

Describe the alternatives you considered
Add a message sending to subscribers in case of failure.
and/or, After the backup try to do a linux command "df $folder --output=avail" "df $folder --output=iavail" and if one or the other has a value too low, send a message to the subscribers

Additional context
Some local backups could not be made due to lack of disk space in the folder provided upon receipt.

If you accept this suggestion, I can work on it.